• uri (canvas URI for create; formula/control URI for update)
• data.action: "create" or "update"
• For create: control.type ("formula" or "button"); for formula: control.formula (CFL) and control.format (full column-format set: canvas, check, curr, dp, dt, email, link, lookup, none, num, per, person, reaction, scale, sl, slider, time); for button: control.actionFormula, control.label, control.color, control.disableIfFormula
• control.name (optional, auto-generated if omitted)
• insertPosition (element ID or "page_start")
• For update: updatedFields { formula, name, format, label, actionFormula, color, disableIfFormula }